Diagnosing a Flashing Brake Warning Light
 

Can someone help me with a step-by-step to diagnosing my flashing brake warning light (1973 1/2 911T)?

Here's the current state and what I've done so far:

- The brake warning light on the oil temp/pressure gauge is flashing. After a period period it was intermittent, it now flashes all the time

- I checked the brake warning switch on the master cylinder, cleaned at the connections as reset

- I checked my brake lights (my wife is not a very willing helper -- she hates my car -- so I had to wait to she was in an exceptionally good mood to ask for help) and they are not working. I checked the bulbs and they are good.

Could it be that the flashing warning light is due to the brake pedal switch having failed? I have not found anything in any of my manuals (owners, Haynes, etc.) that indicates that the warning light flashes when a bulb is out or the pedal switch has failed. Also, I have found no evidence that there is a stop light relay -- is this correct?

Thanks in advance for any help.

Do you have pad wear sensors on your calipers? if so raiser her safely and check the front and rears for wear of the pads.

Intermittent tells me one of the sensors started to go and when it finally cut thru ergo "WARNING"

if you unplug the master cylinder switch wires, i bet it goes away. you would know if you had a brake problem. you don't need a flashing light to tell you that.

Thanks. No wear sensors on my pads (it's a '73).

I'll do the reset of the brake warning switch again and bleed the rear brakes. It's quite a PITA given the gravel guard, etc. Been thinking about replacing the master cylinder if I need a new switch: the MC is the original on the car (39 years old and yet to be rebuilt) but the brakes are solid even at high speed (see below) without even a hint of sponginess or fading. Thoughts? Do master cylinders tend to fail gracefully or catastrophically?

What about the brake lights not working? The brake warning light isn't used to indicate a brake light failure problem as well, is it?

Any idea how to test the brake pedal switch and the circuit from the switch to the brake light (I saw a thread on using jumpers to test the continuity of the circuit, but can't seem to find it now)? Is there a relay in brake light circuit?

I had the rear light housings out so decided to do a light cleaning/renewal of them so probably won't be able do any further troubleshooting for a couple of days.

BTW, had the car out on SH130 -- the new "fastest highway in America". Beautiful road, car was pretty stable, got close to redline at which point the front end got a little light -- next time I'll fill the tank before starting a run up that road . Appropriately, the road goes right past the Circuit of the Americas, where we'll have the first F1 race in the US, in what, 5 or 6 years?

Had the same thing happen to me a few years ago.

As John Walker noted, it stopped when I disconnected the connector from the the switch on the brake master cylinder (it is only for the warning light, not the brake lights). You can buy just the switch and replace it like I did. Once you remove the wheel and gravel guard for the steering rack, it is easily accessible. If you have a firm pedal then you do not need a new cylinder.

FWIW, the switch is an USA only feature and I know for a fact it only comes on a milli second after your foot hits the floor due to a failed hydraulic system (I had it happen in a 1972 Ford Station Wagon when I was on a freeway exit ramp and the car was loaded to it's maximum load).

If you need to test your brake pedal and do not want to ask for help. Get a short stick that you can wedge between the brake pedal and your steering wheel. Jam it in there and you are good to go. It is also handy when you are doing brake work where you need to remove a brake line and do not want to drain your entire brake system.

...my wife hates my car too...

Is this the same light that flashes when you apply the parking brake?

Yes

disconnect and forgetaboutit. turn to the right all the way and reach in behind the tire and pull the wire off.

The flashing light is only for reminding you that the parking brake is set or that your hydraulics have failed.

There is no stop light relay.

I had this on a 69T i collected for someone recently. It turned out to be the handbrake warning light even though the handbrake itself was fine

mmmm, the handbrake warning light is a function I would want to keep. Easy to check, pull the wires off the switch in the rear of the handbrake mechanism. Its a fairly agricultural arrangement with 2 wires, you can adjust the operation to suit.

Pull the connector off of the master cylinder first to elminate it as the source. Chances are it is the master cylinder switch, not the one on the hand brake. The hand brake warning will continue to function.

Thanks for this info. Unplugged mine and no more flashing light!! Yes!

A proper fix is advised.

If a stage of your dual stage master cylinder fails, this light will blink. If the other stage on your master cylinder fails... You might kiss a tree, or worse, an innocent fellow motorist.

Unplugging factory warning lights. Does that really seem like a good thing to do? :D

resurrecting this thread -- on my '86 targa, my brake warning light (not the pad sensor light) is starting to intermittently light up -- generally when making spirited low speed turns or going over the curb bump to my driveway. Doesn't seem to be affected by playing with my handbrake, leading me to believe it's the MC switch that's failing -- I'll eventually replace it, but want to be sure that my symptoms don't clearly point to a looming brake failure.

I would doubt the switch would be failing. More than likely the wire leading to the switch is cracked and weathered. Simple enough to have a helper watch the dash while you go down below and wiggle that wire. They can get brittle in the environment like that. Fluid level good?

might be sensor for low level brake fluid on the master. richard

thanks eric and pros1968 -- have also been having the brake pad wear light come on intermittently throughout my last few sets of brake pads (while brake pads have plenty of wear left) that problem resets if I "reboot" the car, which is likely another indication of brittle cracked/weathered wires -- will get the "boss"/helper to give me a hand with this shortly and will immediately check brake fluid level

wanted to circle back on my situation -- I checked my brake fluid level and found it to be within the "normal" range. After confirming the brake fluid level itself wasn't the culprit, I manipulated the sensor in the reservior cap a few times to make sure it was freely/properly moving. Although I didn't notice anything remarkable, after reinstalling the brake fluid reservoir cap, the brake warning light stopped blinking/flashing and (for the past few weeks) has behaved normally.

FYI